The term “HVAC system” or “HVAC unit” as used throughout this patent application is to be interpreted as covering a system or unit which at its simplest simply draws air in via the filter and distributes this filtered air to the inside of the cab or a system or unit which additional heats and/or conditions the air which is drawn in via the filter.
It is desirable for the filter to be readily accessible in the housing for easy replacement or cleaning. Also the housing needs to be cheap to manufacture and to have the necessary structural rigidity. The housing may also need to have a relatively intricate shape to join with ducting which conducts air to and from the air filter. EP0438076A shows a tractor cab filter arrangement in which air is drawn into the cab via separate filters at each side of the cab. U.S. Pat. No. 4,365,541 shows a cab filter arrangement in which two filters can be accessed via a single service point.